How do we know if a CME is earth directed and when it's going to arrive?

When there are huge explosions on the Sun you don't know immediatly if it has an earth directed CME associated with it. In this article we digg a bit deeper in the methods that you can use so that you can determine when we have an Earth directed CME and when it's going to arrive on Earth.

Detecting CMEs

1. Using the proton flux monitor

On our website you'll find a data plot with the primary >10MeV solar protons, or in short the Proton flux monitor. With huge explosions on the Sun a Solar radiation storm can be triggered. The Solar protons get blown into space with the speed of light and are the first particles to arrive at Earth. So we know verry fast if a Solar radiation storm is triggered and, in a less accurate way, if there is a CME directed to Earth.

Solar radiation storm

2. Using the LASCO/STEREO images

The best way to be sure when a CME is Earth directed are the images taken by the SOHO spacecraft (LASCO C2 and C3 instruments) and the STEREO spacecraft. You'll find the latest images and videos on our website.

On the LASCO images you can immediatly see if the CME is Earth directed or not. A CME is always good visible in the imagery and when it's Earth direct you'll see a full or partial halo CME. So this means it is partial or fully Earth directed.

With the use of the imagery available from the spacecraft the space weather scientists calculate the departure speed and the estimated time of arrival of the CME. So if you want to know when it arrives you'll have to wait on the reports of the space weather scientists. The daily reports are also available on our website.

Keep in mind that the LASCO images are not always in real-time and you'll have to wait several hours to see the images and to know if it's Earth directed or not.

Example of a full halo CME

3. Using the SIDC Cactus Software

The Belgian Solar Influences Data Analysis Center (SIDC) developed the CACTUS program which stands for Computer Aided CME Tracking. It automaticly scans the LASCO imagary to determine wether it's a halo CME or partial halo CME and also determines the lift off speed of the CME. It can take a while before the LASCO images are processed, mostly the LASCO images come available after 6 hours.

4. Using the EPAM Plot

EPAM stands for the Electron, Proton and Alpha Monitor and is an instrument in the ACE satellite that measures the electrons and protons that are send out with the solar wind. It's a very usefull instrument to know if the CME is Earth directed and when it's going to arrive. We'll be making it a bit clear with some EPAM plots.

The EPAM plot, just after a solar flare

When a huge explosion happens on the Sun, electrons and protons are hurled away from the Sun into space. The electrons and protons are pushed out with the solar wind flow. Immediatly afer such an event the EPAM plot shows a fast rise of the electrons which marks the start of the flare. The proton plot in the plot also shows a steady rise. This indicates that a part of the CME is Earth directed.Now we know that a shockfront of a CME is underway but the question remains: when is it going to arrive?

When does the CME arrive?

Determining the impact time by using the lift off speed of the CME

If we know the speed of the CME, we can easily determine when it is going to arrive. With the following table we listed for different CME speed what the average travel time of a CME is. But keep in mind that this is not an exact travel time, keep in mind that any CME can arrive early or later with a margin of sometimes 6 hours.

Determine the impact by using the EPAM monitor

After the solar flare, the EPAM protons keep rising untill the arrival of the CME shock. The first rise in the plot is also called the "ramp up" or "onset" phase. It keeps rising but flattens when the CME gets closer. A few hours before the actual arrival of the CME a new rise of the protons take place, this indicate that the shock front is going to arrive soon. When the EPAM plot peaks, it indicates that the CME has arrived. After the peak the proton level will decline to normal values (unless a new solare flare occures).

On the EPAM plot below you can see it very good. We can see a clear rise in all values after the solar flare with a good ramp up phase followed by the flattening of the values before rising again to indicate the imminent arrival of the CME. At the peak, the shockwave arrives.

The EPAM plot

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